All such dryers discharge dry pressurized air to systems utilizing such air, which systems often include main reservoirs that store the air for use by such systems. Dry air is discharged from the dryers through a check valve, and the valve remains closed when discharge air is not available to open the valve. The purpose of such a valve is to seal an air compressor and air dryer from the pressure contained within the main reservoir when the air compressor is idle and to prevent any loss of main reservoir pressure (MRP) through the compressor. In the first of the above applications there is an objective to make such an air dryer as compact as possible since ample space is not always available for containing dryers.